342 NERVOUS SYSTEM. 
In this portion of the brain the continuation of the central 
cavity is a narrow canal which receives the name cerebral 
aqueduct (aqueductus cerebri) or aqueduct of Sylvius. 
Fic. 138.—VENTRAL SURFACE OF THE BRAIN. 
/-XI/, the twelve cranial nerves in order; J, olfactory bulb; /Z, N. opticus; Z//, 
N. oculomotorius; 7V, N. trochlearis; VY, N. trigeminus (1, mandibular nerve: 2, 
maxillary nerve; 3, ophthalmic nerve; 4, sensory root); V/Z, N. abducens; V//, N. 
facialis; V7Z/, N. acusticus; ZY, N. glossopharyngeus; Y, N. vagus; XJ, N. acces- 
sorius; XZ/, N. hypoglossus. 4, tractus olfactorius; 4, substantia perforata anterior; 
¢, optic chiasma; ¢@, infundibulum; «¢, hypophysis; 7, lobus pyriformis, or tractus 
postrhinalis; g, pedunculi cerebri; 4, substantia perforata posterior; 2, pons; 7, sulcus 
basilaris of pons; 4, semilunar (or Gasserian) ganglion; /, trapezium; 7, area ovalis; 
w, area elliptica; 0, pyramidal tracts; #, anterior or ventral fissure; g, foramen 
cecum; 7, anterior lateral sulcus; s, first cervical nerve. 
Just craniad of the midbrain is a small rather irregular area, 
bounded craniad by the band-like optic chiasma (c), from 
which arise the optic nerves (//), and showing caudad of this 
band a rounded irregular projection (¢). This is a fourth divi- 
sion of the brain which is likewise covered dorsad by the 
hemispheres; it is the ’tween-brain or diencephalon. Its 
cavity is known as the third ventricle. Just craniad of the 
